1. Field of the Invention
The present invention is generally related to the field of pyrolytic carbon coating techniques, and, more particularly, to a method of controlling the coating process and determining the coating rate of pyrolytic carbon on a product positioned in a fluidized bed.
2. Description of the Related Art
It is desirable to deposit pyrolytic carbon coatings on certain objects. For example, uranium particles can be coated with a pyrolytic carbon which, in part, forms a pressure-retentive shell allowing the coated particles to be fabricated into fuel rods for use in nuclear reactors. Another use for such coatings is for heart valves and other biomedical components because a pyrolytic carbon coating does not react with blood. For example, pyrolytic carbon heart valves may be formed by positioning a reverse mold comprised of graphite in a fluidized bed, and forming the carbon material on the mold. Thereafter, the pyrolytic carbon material that comprises the heart valve may be removed from the reverse mold.
Pyrolytic carbon is usually deposited on an object by thermally decomposing gaseous hydrocarbons or other carbonaceous substances in vaporous form in the presence of the object. A pyrolytic carbon deposition process is typically performed in a fluidized bed apparatus. The apparatus is generally comprised of a bed of very small particles (usually of a size measured in microns) positioned within the apparatus, a means for circulating a variety of process gases through the bed of particles, and a means for heating the apparatus, e.g., typically an RF heating coil. The products that are desired to be coated are positioned within the bed of particles. Typically, the products will be relatively large relative to the size of the particles within the bed. This arrangement provides sufficient available total surface area to assure that pyrolytic carbon having the desired crystalline form will be deposited on the product. In addition, the random motion of the relatively large products in the fluidized bed provides for a relatively uniform deposition of the carbon material on all surfaces of the product.
However, whenever such submillimeter particles are being coated in a fluidized bed, the total surface area of the particles begins to increase significantly as the diameters of the pyrolytic carbon-coated particles grow. This change in the available deposition surface area in the fluidized bed affects the coating rate and results in a change in the physical characteristics of the pyrolytic carbon being deposited if the other coating variables, e.g., coating temperature, gas flow rate and gas composition, are held constant. Moreover, when the bed reaches some maximum size, it will collapse and thus limit the thickness of the carbon coating that can be deposited on levitated products within the bed under constant input conditions. Changes in the physical characteristics of the carbon deposited may be undesirable for any of a number of reasons.
As set forth above, the pyrolytic coating process is a relatively complicated process, the effectiveness of which depends upon a variety of interrelated factors. In general, what is desired is to be able to control such a deposition process to insure that the desired amount of pyrolytic carbon material is deposited on the products during the process. For example, with respect to the manufacture of heart valves comprised of pyrolytic carbon, the resulting valves must meet a relatively tight thickness specification. Heart valves that are manufactured too thin are rejected because, among other things, the parts will not provide the necessary structural strength for the completed device. On the other hand, heart valves that are too thick are also rejected because, among other things, the increased thickness results in a heart valve that is too rigid, thereby leading to undesirable stress levels in the heart valve when it is in service. Thus, better thickness control in pyrolytic carbon deposition processes is desired. Other efforts have been made to attempt to control variations in pyrolytic coating processes by attempting to maintain a constant bed area during a coating run. For example, such efforts have included attempts to maintain a constant bed weight, see, e.g., U.S. Pat. No. 5,328,713, and a constant differential pressure across the bed, see, e.g., U.S. Pat. No. 5,514,410. However, to date, thickness control of existing pyrolytic carbon coating techniques and methods is less than desirable.
The present invention is directed to a method and system that may solve, or at least reduce, some or all of the aforementioned problems.
The present invention is generally directed to a method of controlling a pyrolytic carbon deposition coating process performed in a fluidized bed. In one illustrative embodiment, the method comprises positioning a product to be coated in the fluidized bed, determining a coating rate of carbon material formed on the product in the fluidized bed, and determining a desired duration of the coating process by dividing a desired coating thickness for the product by the determined coating rate.
In a further embodiment, the method comprises collecting a quantity of particles from the fluidized bed, the collected particles being coated with a pyrolytic carbon material, determining a weight of the collected particles, and determining a deposition rate of the pyrolytic carbon material on the collected particles. The method further comprises determining a feed rate of additional bed particles added to the fluidized bed, determining a new weight of the fluidized bed based upon A) the initial bed weight, B) the determined deposition rate of the pyrolytic carbon material, C) the determined weight of the collected particles, and D) the feed rate of the additional particles. The method concludes with the steps of calculating a coating rate for a product positioned in the fluidized bed based upon, in part, the determined new weight of the bed, and determining a desired duration of the coating process by dividing a desired coating thickness by the determined coating rate.